In recent years, printed circuit boards (also referred to as printed boards) were developed which comprise an insulating substrate, such as a plastic substrate (including film), a ceramic substrate, or a metal substrate coated with a plastic or other substance, and an electronic circuit formed thereon; and a technique of soldering an electronic part such as an IC element, semiconductor chip, resistor, or capacitor to the circuit surface of such a printed board to constitute an electronic device is being widely employed.
In producing the above-described circuit device having electronic parts mounted thereon, the ordinary method for soldering a lead terminal of the electronic part to a predetermined pad of the circuit comprises forming a thin solder layer beforehand on either or both of the pad and the lead terminal, positioning the electronic part, and then melting (reflowing) the thin solder layer(s) to solder the electronic part on the circuit.
For the formation of the solder circuit (thin solder layer), such methods as plating, dipping (immersion) in a solder bath, and printing of a solder powder paste have been employed. However, as the trend toward the increase in mounting density, solder circuits have been required to have even finer patterns and there also are desires for improvements in working efficiency and on-specification rate and for circuit pattern miniaturization. It is, therefore, becoming difficult to cope with these requirements with the above methods.
Among those conventional methods, the plating method is applicable for formation of solder circuits having highly precise and fine patterns.
The plating method is classified into electroplating and electroless plating. Use of electroplating, however, encounters difficulties in attaining electrical conductivity because, in actual printed circuit boards, the parts in which solder circuits are to be formed are present independently from the circuit parts in most cases. On the other hand, electroless plating has a technical problem that it is difficult to obtain a thick solder layer having a thickness necessary in practical use, although the problem concerning electrical conductivity in the electroplating is overcome.
A method of electrostatically applying flux-coated solder powder particles on a circuit part has been proposed in JP-A-3-50853. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, this method is still unable to easily produce a high-accuracy fine pattern.
Another method for the solder circuit formation has been proposed in JP-A-4-10694, which comprises applying a flux on a circuit part by printing, adhering a powdered solder to the flux-printed part, melting the solder by heating it to a temperature not lower than the melting point of the solder, and then blowing a gas on the solder melt to level it to thereby form a solder circuit. This method is disadvantageous in that a high degree of skill is required because high-precision printing of a flux on a pad is difficult and in addition there is a fear of bridging between patterns spaced at a minute gap during the leveling of the solder melt.
Also in the case where a solder coat (solder layer) is formed on a lead terminal of an electronic part, there are the same technical problems as those in the formation of solder circuits.
As a result of extensive studies made in order to improve the precision of solder patterns which are required to be fine, it has been found that the plating method, which has attained the highest precision, still has several problems and has to be improved in working efficiency and other respects.